Emails Not Sending!MailClient.Storage.Application.ItemOperationException: Cannot send message (Email recipient not accepted by the server.)

1

Email wont send. Error about server. MailClient.Storage.Application.ItemOperationException: Cannot send message (Email recipient not accepted by the server.)  
Very frustrated. Have tried reinstalling, restarting computer, shutting down computer, deleting emails that were not sending, etc. Only wont send from your application, emails go out from my phone and email client when sending direct just not your application on my laptop!

2

just going to go back to using previous email client.  Thanks